Mercury remains above 45 degrees Celsius in many parts of Delhi on Monday

There was no let-up in the heat wave that has gripped the national capital, with the mercury staying above the 45-degree Celsius mark in many areas on Monday. Weather experts said a fresh western disturbance may bring some relief on the weekend.

At the Safdarjung Observatory, Delhi's base station, the maximum temperature settled at 43.5 degrees Celsius as against 44.2 degrees Celsius on Sunday and 43.9 on Saturday.

The maximum temperature settled at 46.4 degrees, five notches above normal, at Najafgarh, making it the hottest place in the capital.

Sports Complex, Pitampura, and Jafarpur recorded a high of 46.1 degrees Celsius, 45.8 degrees Celsius and 46 degrees Celsius, respectively.

The Met Office has issued a yellow alert, warning of heatwave conditions at isolated places in the capital on Tuesday.

The India Meteorological Department (IMD) uses four colour codes for weather warnings -- green (no action needed), yellow (watch and stay updated), orange (be prepared) and red (take action).

Mahesh Palawat, vice-president (climate change and meteorology), Skymet Weather, attributed the heatwave spell to the lack of strong western disturbances and incessant hot and dry westerly winds.

He said a fresh western disturbance may induce a cyclonic circulation over Punjab and Haryana which would lead to intermittent pre-monsoon activity in Haryana, Punjab, north Rajasthan and west Uttar Pradesh from June 10.

The maximum temperature in the capital may drop to 40-41 degrees Celsius by Friday.

"With the monsoon expected to cover eastern India by June 15, easterly winds will bring in moisture and intensify pre-monsoon activity in northwest India," he said.

A heat wave is declared when the maximum temperature is over 40 degrees Celsius and at least 4.5 notches above normal. A severe heatwave is declared if the departure from normal temperature is by more than 6.4 notches, according to the IMD.

Based on the absolute recorded temperatures, a heatwave is declared when an area logs a maximum temperature of 45 degrees Celsius.

A severe heatwave is declared if the maximum temperature crosses the 47-degree Celsius mark.